Maria Celeste M. Ramirez

Mutations in Capillary Morphogenesis Gene-2 Result in the Allelic Disorders Juvenile Hyaline Fibromatosis and Infantile Systemic Hyalinosis

Juvenile hyaline fibromatosis (JHF) and infantile systemic hyalinosis (ISH) are autosomal recessive syndromes of unknown etiology characterized by multiple, recurring subcutaneous tumors, gingival hypertrophy, joint contractures, osteolysis, and osteoporosis. Both are believed to be allelic disorders; ISH is distinguished from JHF by its more severe phenotype, which includes hyaline deposits in multiple organs, recurrent infections, and death within the first 2 years of life. Using the previously reported chromosome 4q21 JHF disease locus as a guide for candidate-gene identification, we identified and characterized JHF and ISH disease-causing mutations in the capillary morphogenesis factor-2 gene (CMG2). Although CMG2 encodes a protein upregulated in endothelial cells during capillary formation and was recently shown to function as an anthrax-toxin receptor, its physiologic role is unclear. Two ISH family-specific truncating mutations, E220X and the 1-bp insertion P357insC that results in translation of an out-of-frame stop codon, were generated by site-directed mutagenesis and were shown to delete the CMG-2 transmembrane and/or cytosolic domains, respectively. An ISH compound mutation, I189T, is predicted to create a novel and destabilizing internal cavity within the protein. The JHF family-specific homoallelic missense mutation G105D destabilizes a von Willebrand factor A extracellular domain alpha-helix, whereas the other mutation, L329R, occurs within the transmembrane domain of the protein. Finally, and possibly providing insight into the pathophysiology of these diseases, analysis of fibroblasts derived from patients with JHF or ISH suggests that CMG2 mutations abrogate normal cell interactions with the extracellular matrix.

Mutations in PDGFRB cause autosomal-dominant infantile myofibromatosis.

Infantile myofibromatosis (IM) is a disorder of mesenchymal proliferation characterized by the development of nonmetastasizing tumors in the skin, muscle, bone, and viscera. Occurrence within families across multiple generations is suggestive of an autosomal-dominant (AD) inheritance pattern, but autosomal-recessive (AR) modes of inheritance have also been proposed. We performed whole-exome sequencing (WES) in members of nine unrelated families clinically diagnosed with AD IM to identify the genetic origin of the disorder. In eight of the families, we identified one of two disease-causing mutations, c.1978C>A (p.Pro660Thr) and c.1681C>T (p.Arg561Cys), in PDGFRB. Intriguingly, one family did not have either of these PDGFRB mutations but all affected individuals had a c.4556T>C (p.Leu1519Pro) mutation in NOTCH3. Our studies suggest that mutations in PDGFRB are a cause of IM and highlight NOTCH3 as a candidate gene. Further studies of the crosstalk between PDGFRB and NOTCH pathways may offer new opportunities to identify mutations in other genes that result in IM and is a necessary first step toward understanding the mechanisms of both tumor growth and regression and its targeted treatment.

Morbid Obesity Resulting from Inactivation of the Ciliary Protein CEP19 in Humans and Mice

Obesity is a major public health concern, and complementary research strategies have been directed toward the identification of the underlying causative gene mutations that affect the normal pathways and networks that regulate energy balance. Here, we describe an autosomal-recessive morbid-obesity syndrome and identify the disease-causing gene defect. The average body mass index of affected family members was 48.7 (range = 36.7-61.0), and all had features of the metabolic syndrome. Homozygosity mapping localized the disease locus to a region in 3q29; we designated this region the morbid obesity 1 (MO1) locus. Sequence analysis identified a homozygous nonsense mutation in CEP19, the gene encoding the ciliary protein CEP19, in all affected family members. CEP19 is highly conserved in vertebrates and invertebrates, is expressed in multiple tissues, and localizes to the centrosome and primary cilia. Homozygous Cep19-knockout mice were morbidly obese, hyperphagic, glucose intolerant, and insulin resistant. Thus, loss of the ciliary protein CEP19 in humans and mice causes morbid obesity and defines a target for investigating the molecular pathogenesis of this disease and potential treatments for obesity and malnutrition.

Hyaline Fibromatosis Syndrome inducing mutations in the ectodomain of anthrax toxin receptor 2 can be rescued by proteasome inhibitors

Hyaline Fibromatosis Syndrome (HFS) is a human genetic disease caused by mutations in the anthrax toxin receptor 2 (or cmg2) gene, which encodes a membrane protein thought to be involved in the homeostasis of the extracellular matrix. Little is known about the structure and function of the protein or the genotype–phenotype relationship of the disease. Through the analysis of four patients, we identify three novel mutants and determine their effects at the cellular level. Altogether, we show that missense mutations that map to the extracellular von Willebrand domain or the here characterized Ig‐like domain of CMG2 lead to folding defects and thereby to retention of the mutated protein in the endoplasmic reticulum (ER). Mutations in the Ig‐like domain prevent proper disulphide bond formation and are more efficiently targeted to ER‐associated degradation. Finally, we show that mutant CMG2 can be rescued in fibroblasts of some patients by treatment with proteasome inhibitors and that CMG2 is then properly transported to the plasma membrane and signalling competent, identifying the ER folding and degradation pathway components as promising drug targets for HFS.

Infantile systemic hyalinosis: Case report and review of the literature.

Infantile systemic hyalinosis (ISH) is a rare, progressive autosomal recessive disease, which is usually fatal by the age of 2 years. Clinical onset typically occurs within the first few weeks of life. The disease is characterized by joint contractures, osteopenia, failure to thrive, gingival hypertrophy, diarrhea, protein-losing enteropathy, and frequent infections. Dermatologic manifestations include thickened skin, hyperpigmentation, perianal nodules, and facial papules. Histopathology shows hyaline deposits in the dermis and visceral organs. We describe a patient with ISH confirmed by clinical and histopathologic findings, as well as DNA sequence analysis, which revealed a novel homozygous T118K mutation in the CMG2 gene.

Systemic hyalinosis mutations in the CMG2 ectodomain leading to loss of function through retention in the endoplasmic reticulum.

Systemic hyalinosis is an autosomal recessive disease that encompasses two allelic syndromes, infantile systemic hyalinosis (ISH) and juvenile hyaline fibromatosis (JHF), which are caused by mutations in the CMG2 gene. Here we have analyzed the cellular consequences of five patient‐derived point mutations in the extracellular von Willebrand domain or the transmembrane domain of the CMG2 protein. We found that four of the mutations led to retention of the protein in the endoplasmic reticulum (ER), albeit through different mechanisms. Analysis of recombinant CMG2 von Willebrand factor A (vWA) domains, to which three of the mutations map, indicated that the mutations did not prevent proper folding and ligand binding, suggesting that, in vivo, slow folding, rather than misfolding, is responsible for ER retention. Our work shows that systemic hyalinosis can be qualified as a conformational disease, at least for the mutations that have been mapped to the extracellular and transmembrane domains. The long ER half‐life and the ligand binding ability of the mutated von Willebrand domains suggest that treatments based on chemical chaperones could be beneficial. Hum Mutat 0, 1–7, 2009.

Uroporphyrinogen III Synthase Knock-In Mice Have the Human Congenital Erythropoietic Porphyria Phenotype, Including the Characteristic Light-Induced Cutaneous Lesions

Congenital erythropoietic porphyria (CEP), an autosomal recessive inborn error, results from the deficient but not absent activity of uroporphyrinogen III synthase (URO-synthase), the fourth enzyme in the heme biosynthetic pathway. The major clinical manifestations include severe anemia, erythrodontia, and disfiguring cutaneous involvement due to the accumulation of phototoxic porphyrin I isomers. Murine models of CEP could facilitate studies of disease pathogenesis and the evaluation of therapeutic endeavors. However, URO-synthase null mice were early embryonic lethals. Therefore, knock-in mice were generated with three missense mutations, C73R, V99A, and V99L, which had in vitro-expressed activities of 0.24%, 5.9%, and 14.8% of expressed wild-type activity, respectively. Homozygous mice for all three mutations were fetal lethals, except for mice homozygous for a spontaneous recombinant allele, V99A(T)/V99A(T), a head-to-tail concatemer of three V99A targeting constructs. Although V99A(T)/V99A(T) and C73R/V99A(T) mice had approximately 2% hepatic URO-synthase activity and normal hepatic microsomal heme and hemoprotein levels, they had 20% and 13% of wild-type activity in erythrocytes, respectively, which indicates that sufficient erythroid URO-synthase was present for fetal development and survival. Both murine genotypes showed marked porphyrin I isomer accumulation in erythrocytes, bone, tissues, and excreta and had fluorescent erythrodontia, hemolytic anemia with reticulocytosis and extramedullary erythropoiesis, and, notably, the characteristic light-induced cutaneous involvement. These mice provide insight into why CEP is an erythroid porphyria, and they should facilitate studies of the disease pathogenesis and therapeutic endeavors for CEP.

Abstract LB-89: Discovery and characterization of novel MTAP splice variants resulting in a hereditary form of osteosarcoma and demonstration of their dysregulation in sporadic forms of this cancer

Proceedings: AACR 103rd Annual Meeting 2012‐‐ Mar 31‐Apr 4, 2012; Chicago, IL Hereditary cancer syndromes represent a powerful and tractable biologic system for identifying cancer-causing genes. Though the syndromes themselves may be rare, their study can provide insights into the basis of the more common sporadic forms of the cancer. Diaphyseal medullary stenosis with malignant fibrous histiocytoma (DMS-MFH) is an autosomal dominant bone dysplasia / bone cancer syndrome. This hereditary cancer syndrome is characterized by bone infarctions, cortical growth abnormalities, and pathologic fractures. Most notably, 35% of affected individuals develop bone MFH, a sarcoma that in its sporadic form accounts for 6% of all bone cancers and is believed to be etiologically related to osteosarcoma. Indeed, one of our affected family members developed histologically-proven osteosarcoma, thus further supporting a genetic link between these tumor types. Using a linkage based approach, we previously mapped the DMS-MFH tumor suppressor gene locus to chromosome 9p21-22 (1) and then through LOH analysis demonstrated that hereditary and sporadic forms of MFH most likely share a single underlying genetic etiology (2). We now demonstrate that DMS-MFH results from mutations in the most proximal of three previously unrecognized terminal exons of the methylthioadenosine phosphorylase (MTAP) gene. MTAP is a ubiquitously expressed homotrimeric-subunit enzyme critical to polyamine metabolism and adenine/methionine salvage pathways and was believed to be encoded as a single transcript from the eight previously described exons. Intriguingly, two of the novel MTAP exons arose from early and independent retroviral integration events in primate genomes at least 40 MYR ago and since their genomic integration have gained a functional role. Six distinct retroviral-sequence containing MTAP isoforms, each of which can physically interact with archetype MTAP (i.e., exons 1-8), are identified. The disease-causing / cancer-associated mutations occur within one of these retroviral-derived exons. The mutations result in exon skipping and dysregulated alternative splicing of all MTAP isoforms. Based on these findings in a hereditary form of bone sarcoma, we then analyzed the expression of these MTAP isoforms in a sample set (n=16) of sporadic osteosarcoma samples. All tumor samples expressed similar levels of the archetype MTAP RNA sequence but the expression pattern of the splice variants varied markedly between nearly all the samples. The majority of samples did not express SV1 (n=11/16) and nearly half did not express SV6 (n=9/16). Taken together, these results identify the first gene involved in the development of bone MFH / osteosarcoma and have potential implications for the treatment of this human cancer. References: 1. Am J Hum Genet. 64:801-7; 1999. 2. Genes Chromosomes Cancer. 27:191-5; 2000. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr LB-89. doi:1538-7445.AM2012-LB-89